The dissociation constant indicates the strength of acids and bases, defined by their ability to transfer protons or produce hydroxide ions in aqueous solutions. The acid dissociation constant (Ka) and base dissociation constant (Kb) are significant for understanding acid-base equilibria and are often represented as pKa. Measurement techniques for these constants include potentiometric titration, spectrophotometry, and more.

1. Dissociation Constant

2.  The strength of an acid depends on its ability to
transfer its proton (H+) to
A Base to form a conjugated base.
When an acid (HA) dissolves in water
It transfers its proton to water (as bronsted base)
to form hydronium ion (H3O+) and conjugated
base.
HA + H2O ⥨ H3O+ + A-
Conjugate
base

3.  HA + H2O ⥨ H+ + A-
 This equation represent the
dissociation of HA into H+ and A-
Applying law of mass action
Where Ka is called the acid dissociation
constant.

4. Similarly for bases
 According to a Arrhenius concept,
base is a substance produce OH- ions.
 Let us consider base BOH
BOH ⥨ B++OH-
Applying law of mass action
 Where Kb is called base dissociation
constant.

5.  So, strength of an acid and base is
defined as the concentration of the H+
ions and OH- ions, respectively in its
aqueous solution at room temperature.
 So, value of dissociation constant (for
both acid and base) measures its acid
strength and base strength or acidity and
basicity respectively.
 The most common way to represent the
dissociation constant by taking -Log10 of
dissociation constant.
 Represented as pKa
 pKa = -log10 (Ka).

6. Applications
1. The acid base dissociation constant
(pka) of a drug is a physiochemical
parameter which influences
biopharmaceutical properties.
2. The knowledge of the pka value is
important for quantitative analysis of
system involving acid base
equilibrium.
3. It is also essential for working with
buffers.
4. It is used with pH indicators.

7. Measurement of dissociation
constant
 Ka and kb or pka can be determined
by:
1. Potentiometric titration.
2. Spectrophotometric methods.
3. Capillary electrophoresis.
4. Liquid chromatography.
5. Conductivity method.
6. Solubility method.

8. Thank you